Antibody synthesis is a defense response of higher vertebrates. The molecular entities which stimulate antibody synthesis (e.g., a virus particle) are called antigens. The introduction of an antigen into the body of a higher vertebrate stimulates specific white blood cells, B lymphocytes, to produce antibodies that combine specifically with the antigen to prevent its further multiplication, or to otherwise inactivate it. The study of antibodies and their action with antigens is a branch of immunology.
Antibodies which circulate in blood or other body fluids are termed humoral antibodies, as distinguished from "membrane antibodies" which remain bound to their parent lymphocytes. The term immunoglobulin is used to generically refer to all antibodies. In humans, all immunoglobulins are divided into five classes termed IgG, IgA, IgM, IgD and IgE. Each immunoglobulin molecule consists of two pairs of identical polypeptide chains. The larger pair termed "heavy chains" and designated gamma (.gamma.), alpha (.alpha.), mu (.mu.), delta (.delta.) and epsilon (.epsilon.), respectively, are unique for each immunoglobulin class and are linked together by disulfide (s--s) bonds between each chain. Each heavy chain consists of about 400 to 500 amino acid residues linked together by polypeptide bonds. Each light chain, by contrast, consists of about 200 amino acids and are usually linked to a heavy chain by a single disulfide bond.
In 1969, Gerald Edelman first determined the amino acid sequence of an entire human IgG molecule. He found that both heavy and light chains are organized into homology units or "domains" about 100 amino acids in length. Subsequent sequence analysis of the other four immunoglobulin classes demonstrate that they are also organized into structurally similar domains having different amino acid sequences. The first or amino-terminal domain of both light and heavy chains have discrete regions within which considerable variation in amino acids occur. These domains are therefore termed variable (V) domains and are designated V.sub.H in heavy chains and V.sub.L in light chains.
The molecular association of a V.sub.L and V.sub.H domain within an intact immunoglobulin forms an antigen combining site which may bind to a specific antigen with high affinity. The domain structure of all light chains is identical regardless of the associated heavy chain class. Each light chain has two domains, one V.sub.L domain and one domain with a relatively invariant amino acid sequence termed constant, light or C.sub.L.
Heavy chains, by contrast may have either three (IgG, IgA, IgD) or four (IgM, IgE) constant or C domains termed C.sub.H 1, C.sub.H 2, C.sub.H 3, and C.sub.H 4 and one variable domain, termed V.sub.H. Alternatively, C domains may be designated according to their heavy chain class; thus C.sub..epsilon. 4 indicates the C.sub.H 4 domain of the IgE (epsilon) heavy chain.
Visualization of antibodies by electron microscopy or by X-ray diffraction reveals that they have a "Y" shape. IgA and IgM antibodies, in addition, combine in groups of two and five, respectively, to form dimers and pentamers of the basic Y shaped antibody monomer.
When antibodies are exposed to proteolytic enzymes such as papain or pepsin, several major fragments are produced. The fragments which retain antigen binding ability consist of the two "arms" of the antibody's Y configuration and are termed Fab (fragment-antigen binding) or Fab'2 which represent two Fab arms linked by disulfide bonds. The other major fragment produced constitutes the single "tail" or central axis of the Y and is termed Fc (fragment-crystalline) for its propensity to crystallize from solution. The Fc fragment of IgG, IgA, IgM, and IgD consists of dimers of the two carboxy terminal domains of each antibody (i.e., C.sub.H 2 and C.sub.H 3 in IgG, IgA and IgD, and C.sub.H 3 and C.sub.H 4 in IgM.) The IgE Fc fragment, by contrast, consists of a dimer of its three carboxy-terminal heavy chain domains (C.sub..epsilon. 2, C.sub..epsilon. 3 and C.sub..epsilon. 4).
The Fc fragment contains the antibody's biologically "active sites" which enable the antibody to "communicate" with other immune system molecules or cells and thereby activate and regulate immune system defensive functions. Such communication occurs when active sites within antibody regions bind to molecules termed Fc receptors.
Fc receptors are molecules which bind with high affinity and specificity to molecular active sites with immunoglobulin Fc regions. Fc receptors may exist as integral membrane proteins within a cell's outer plasma membrane or may exist as free, "soluble" molecules which freely circulate in blood plasma or other body fluids.
Each of the five antibody classes have several types of Fc receptors which specifically bind to Fc regions of a particular class and perform distinct functions. Thus IgE Fc receptors bind with high affinity to only IgE Fc regions or to isolated IgE Fc fragments. It is known that different types of class specific Fc receptors exist which recognize and bind to different locations within the Fc region. For example, certain IgG Fc receptors bind exclusively to the second constant domain of IgG (C.sub.H 2), while Fc receptors mediating other immune functions bind exclusively to IgG's third constant domain (C.sub.H 3). Other IgG Fc receptors bind to active sites located in both C.sub.H 2 and C.sub.H 3 domains and are unable to bind to a single, isolated domain.
Once activated by antibody Fc region active sites, Fc receptors mediate a variety of important immune killing and regulatory functions. Certain IgG Fc receptors, for example, mediate direct killing of cells to which antibody has bound via its Fab arms (antibody-dependent cell mediate cytotoxicity--(ADCC)). Other IgG Fc receptors, when occupied by IgG, stimulate certain white blood cells to engulf and destroy bacteria, viruses, cancer cells or other entities by a process known as phagocytosis Fc receptors on certain types of white blood cells known as B lymphocytes regulate their growth and development into antibody-secreting plasma cells. Fc receptors for IgE located on certain white cells known as basophils and mast cells, when occupied by antigen bridged IgE, trigger allergic reactions characteristic of hayfever and asthma.
Certain soluble Fc receptors which are part of the blood complement system trigger inflammatory responses able to kill bacteria, viruses and cancer cells. Other Fc receptors stimulate certain white blood cells to secrete powerful regulatory or cytotoxic molecules known generically as lymphokines which aid in immune defense. These are only a few representative examples of the immune activities mediated by antibody Fc receptors.
It is only after antibodies bind to antigen or are otherwise caused to aggregate that active sites within the Fc region are able to bind to and activate Fc receptors. Fc receptors are, therefore, the critical link between antibodies and the remainder of the immune system. Fc receptor binding to antibody Fc region active sites may thus be characterized as the "final common pathway" by which antibody functions are mediated. If an antigen bound antibody does not bind to an Fc receptor, the antibody is unable to activate the other portions of the immune system and is therefore rendered functionally inactive.
Any peptide with the ability to bind to immunoglobulin Fc receptors has therapeutic usefulness as an immunoregulator by virtue of the peptide's ability to regulate binding to the receptor. Such an Fc receptor "blocker" occupies the immunoglobulin binding site of the Fc receptor and thus "short circuits" the immunoglobulin's activating ability.